A brief History of Time pdfdrive com

THE EXPANDING
UNIVERSE
f one looks at the sky on a clear, moonless night, the brightest objects
one sees are likely to be the planets Venus, Mars, Jupiter, and Saturn.
There will also be a very large number of stars, which are just like our
own sun but much farther from us. Some of these fixed stars do, in fact,
appear to change very slightly their positions relative to each other as
the earth orbits around the sun: they are not really fixed at all! This is
because they are comparatively near to us. As the earth goes round the
sun, we see them from different positions against the background of
more distant stars. This is fortunate, because it enables us to measure
directly the distance of these stars from us: the nearer they are, the more
they appear to move. The nearest star, called Proxima Centauri, is found
to be about four light-years away (the light from it takes about four
years to reach earth), or about twenty-three million million miles. Most
of the other stars that are visible to the naked eye lie within a few
hundred light-years of us. Our sun, for comparison, is a mere eight light-
minutes away! The visible stars appear spread all over the night sky, but
are particularly concentrated in one band, which we call the Milky Way.
As long ago as 1750, some astronomers were suggesting that the
appearance of the Milky Way could be explained if most of the visible
stars lie in a single disklike configuration, one example of what we now
call a spiral galaxy. Only a few decades later, the astronomer Sir William
Herschel confirmed this idea by painstakingly cataloging the positions
and distances of vast numbers of stars. Even so, the idea gained complete
acceptance only early this century.
Our modern picture of the universe dates back to only 1924, when the
American astronomer Edwin Hubble demonstrated that ours was not the
only galaxy. There were in fact many others, with vast tracts of empty
space between them. In order to prove this, he needed to determine the
distances to these other galaxies, which are so far away that, unlike

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nearby stars, they really do appear fixed. Hubble was forced, therefore,
to use indirect methods to measure the distances. Now, the apparent
brightness of a star depends on two factors: how much light it radiates
(its luminosity), and how far it is from us. For nearby stars, we can
measure their apparent brightness and their distance, and so we can
work out their luminosity. Conversely, if we knew the luminosity of stars
in other galaxies, we could work out their distance by measuring their
apparent brightness. Hubble noted that certain types of stars always
have the same luminosity when they are near enough for us to measure;
therefore, he argued, if we found such stars in another galaxy, we could
assume that they had the same luminosity—and so calculate the distance
to that galaxy. If we could do this for a number of stars in the same
galaxy, and our calculations always gave the same distance, we could be
fairly confident of our estimate.
In this way, Edwin Hubble worked out the distances to nine different
galaxies. We now know that our galaxy is only one of some hundred
thousand million that can be seen using modern telescopes, each galaxy
itself containing some hundred thousand million stars.
Fig. 3.1
shows a
picture of one spiral galaxy that is similar to what we think ours must
look like to someone living in another galaxy. We live in a galaxy that is
about one hundred thousand light-years across and is slowly rotating;
the stars in its spiral arms orbit around its center about once every
several hundred million years. Our sun is just an ordinary, average-sized,
yellow star, near the inner edge of one of the spiral arms. We have
certainly come a long way since Aristotle and Ptolemy, when we thought
that the earth was the center of the universe!

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